Various types of boom assemblies for use in cranes or other lifting devices are known in the art. Generally, conventional mobile cranes have an extendable boom assembly including a plurality of telescoping boom sections. Oftentimes, a folded boom extension is stowed alongside the boom assembly. This extension is capable of pivoting into an operative position at the head end of the boom assembly to extend the effective length of the boom assembly, or to change the angular relationship between the head end of the boom assembly and the corresponding hoist line used to lift an object.
An example of this approach is shown in U.S. Pat. No. 4,491,229. In this approach, the boom extension includes a fly section that may be stowed alongside the boom assembly and pivoted to an operative position so as to extend the boom assembly. In the stowed position, an associated jib or “tip” section underlies the base or “fly” section, and in use swings outwardly with the fly section to an aligned position with the boom assembly. If desired, the fly section may be mounted to the head end of the boom assembly while the jib section remains in the stowed position.
While this arrangement accomplishes the goal of selectively increasing the effective length of the boom assembly, it is not without limitation. As one example, the fly and jib sections are quite similar in length, which seriously limits the versatility of this arrangement. Specifically, it is not possible to achieve a shorter lift point using of the fly section only or the combination of the fly section and jib section. In addition, when the fly section either alone or in combination with the jib section is connected to the main boom assembly, the main boom capacity is diminished. Finally, the fly section does not enable the simultaneous use of multiple hoist lines.
Accordingly, a need is identified for a boom extension arrangement that addresses and overcomes the foregoing limitations. Specifically, it is desirable to provide a relatively short “fly” extension stored on the side of the telescopic boom and moved into use without the need for erecting the full extension section, thereby providing a shorter lift point and maximizing the lifting capacity. Compared to past proposals for bringing a separate adaptor section on site, the resulting system would be somewhat less complicated in design, less expensive to manufacture, and easier to install and use.